The present invention relates to paging protocols and more particularly to paging protocols for use in multi-sectorial paging systems wherein not all sectors are illuminated simultaneously.
FLEX.TM. is a well-known paging protocol developed by Motorola, Inc. of Schaumburg, Ill. intended for use with omnidirectional antennas at a plurality of pager transmitter sites. Previous implementations of the FLEX protocol assume that all of the pager transmitter sites broadcast the same signal simultaneously at the same frequency. This style of simultaneous broadcasting is known as simulcast.
In accordance with the FLEX protocol, most of the pagers are in an inactive or sleep mode to conserve energy and increase battery life while the single pager or pagers that are being addressed actively receive and decode messages. The pagers are programmed to wake up periodically in order to check if they have messages, to receive messages, and to maintain their synchronization with the system.
In FLEX, broadcasting time is divided into four minute cycles and each four minute cycle is divided into 128 1.875 second frames. There is a programmable interval between frames for which a pager awakens. There are trade-offs in determining this programmable interval. When the interval is long, energy conservation and thus battery life are improved. However, long intervals are associated with increased latency in receiving messages. In conventional FLEX practice, the interval between frames is N=2.sup.n where n is an integer referred to the collapse value and ranges between 0 and 7. Each pager has its own collapse value which may be user selected to optimize battery life and message latency. However, there is also a "system maximum collapse value," m, determined for the pager network as a whole. If m is less than n for a particular pager, the pager will wake up every N=2.sup.m frames. If n is less than m, the pager will wake up every N=2.sup.n frames.
To distribute pager wake up frames in time and thereby optimize system throughput, each pager is assigned a base frame in the set of 128 frames. The pager wakes up for its base frames and for subsequent frames as determined by the current collapse value. For example, a pager with collapse value of 4 and frame 1 as its base frame will wake up at frames 1, 17, 33, 49, 65, 81, 97, 113, 1, 17, etc. A pager with collapse value of 5 and frame 6 as its base frame will wake up at frames 6, 38, 70, 102, etc.
In the prior art paging system described here, all pagers are accessible during every frame. The assignee of the present application has developed a multi-beam transmission system wherein different beams radiated from a common transmission site carry disparate signals on the same frequency, greatly enhancing system capacity. The system is described in U.S. patent application Ser. No. 08/808,347. One limitation is that adjacent beams generally cannot carry disparate signals simultaneously on the same frequency without causing destructive interference. Therefore, in a pager application, the transmission site will not illuminate all beams simultaneously. It can be appreciated then that the wake-up frame of a pager may not necessarily coincide with the illumination period of the beam servicing the pager.
A similar problem arises in a cellular paging system where there are restrictions on simultaneous transmissions of disparate signals on the same frequency by transmission sites servicing adjacent cells. Again, there is no guarantee that the transmission site servicing a particular pager will be active during the pager's wake-up frame. There is thus a basic incompatibility between the conventional application of an omnidirectional pager protocol that keeps pagers off during certain time periods to save power and a transmission system that increases capacity by spatial multiplexing but places restrictions on the time periods when any particular pager may be addressed. What is needed is a pager protocol that provides the power consumption advantages gained by keeping pagers off except when they need to be addressed but can nonetheless function in an environment where particular geographic sectors are not continuously illuminated.